The present invention relates generally to methods and apparatus for measuring current, and more particularly to current measurement techniques in switching voltage regulators.
Electronic devices are often configured to measure the current passing through a load, which may range in complexity from a single circuit element to a VLSI chip. For example, a fault protection system may measure and limit the amount of current passing through a circuit element to avoid burnout. As another example, a feedback system may measure the current passing through a circuit element, and use this current measurement to control the output of the device.
In conventional current sensors, a resistor having a known resistance R is placed in series with the circuit element to be monitored. The voltage drop V across the resistor is measured, and the current I passing through the circuit element may be calculated from Ohm's Law, V=IR. Unfortunately, special techniques are needed to fabricate resistors in semiconductor devices, thereby increasing the cost of the device or requiring the resistor to be located off the chip as an external component. In addition, the resistor dissipates power, thereby reducing the efficiency of the device.
One particular device which may require a current sensor is a switching voltage regulator (or simply "switching regulator"), such as a DC to DC converter. The switching regulator includes a switch, such as a transistor, for alternately coupling and decoupling an unregulated input DC voltage source, such as a battery, to a load, such as an integrated circuit. An output filter, typically including an inductor and a capacitor, is coupled between the input voltage source and the load to filter the output of the switch and thus provide the output DC voltage. A feedback system measures the current passing through the load, and generates a control signal which controls the duty cycle of the switch in order to maintain the output voltage at a substantially uniform level.